1. Field of the Invention
The present invention relates to an image sensor (e.g., a CMOS sensor) that includes a semiconductor device (e.g., an amplifier) and can cancel the offset voltage of the semiconductor device with a simple circuit configuration.
2. Description of the Related Art
Although a charged coupled device (CCD) is conventionally used widely in an image sensor, an image sensor using a complementary metal oxide semiconductor (hereinafter, “CMOS sensor”) is also widely used recently. Since the CMOS sensor has lower power consumption as compared to the CCD and the already wide spread transistor manufacturing process can be applied to the CMOS sensor, costs can be constrained to a lower level.
Generally, the CMOS sensor is constituted by a pixel unit, a correlated double sampling (CDS) unit, and an analog-to-digital converter (ADC) unit. The pixel unit reads light with a light-sensitive element such as a photo diode (PD), converts the light to an electric signal corresponding to the level of the light quantity and outputs the electric signal.
The CDS unit samples an output level in the case of an offset state without the signal input from the pixel unit and an output level when the electric signal is output to compare two output levels. By obtaining the difference of the comparison, variations in the output value are canceled in each semiconductor device constituting the pixel unit to detect only the pure electric signal.
The ADC unit converts the electric signal input from the CDS unit from an analog signal to a digital signal and outputs the signal as a detected value of one pixel. Recently, to improve a detection speed, a column-type CMOS sensor with a plurality of the above-described COMS sensors arranged in a row has been disclosed (e.g., Japanese Patent Application Laid-Open Nos. 2001-128070 and 2002-218324).
However, in the case of a column-type CMOS sensor, it is problematic that variations in a detected value of each CMOS sensor are displayed as vertical stripes on an image unless variations among the CMOS sensors arranged in the same column.
Although variations in the output of the pixel unit are canceled by the CDS unit in the case of CMOS sensors of Japanese Patent Application Laid-Open Publication Nos. 2001-128070 and 2002-218324 described above, since variations are actually generated due to offsets in a semiconductor device such as an amplifier arranged at a stage after the CDS unit as well as in a semiconductor device such as a comparator of the ADC unit, it is problematic that the variations are displayed as vertical stripes on an image.
For example, if it is configured such that the amplifier is not arranged to prevent the variations due to the amplifier arranged at a stage after the CDS unit, the output of the CDS unit is capacitively divided by a semiconductor device such as a condenser provided in the ADC unit and problematically attenuated to the extent that cannot drive the ADC unit.
On the other hand, in Japanese Patent Application Laid-Open Nos. 2001-128070 and 2002-218324, if a cancel function is added to constrain the variations generated in the semiconductor device such as the amplifier and the comparator at a stage after the CDS unit, since a circuit configuration is made complicated and a circuit scale is increased, it is problematic that the manufacturing cost is increased.